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A severe weather watch, warning of the risk of heavy bursts of rain, has been issued today by the MetService for parts of the North Island from Taranaki and Taihape, northwards to Auckland.

The weather watch, issued this morning, says heavy rain is also expected to affect the Coromandel Peninsula and eastern Bay of Plenty.

"A period of heavy rain is expected from about midday today to Sunday morning.

"There is still uncertainty regarding the timing and exact location of the largest rainfall accumulations. However, there is a risk that rainfall amounts could reach short duration warning criteria -- for example 50mm falling within 6 hours -- during this period, especially about north Taranaki, Waitomo and Taumarunui.

"In addition, there is a risk that rainfall intensities could briefly reach 40mm in one hour. A severe thunderstorm watch has been issued to cover the possibility of localised downpours from Waikato to North Taranaki."

Tomorrow, a period of heavy rain is likely on the Coromandel Peninsula and eastern Bay of Plenty. Rainfall accumulations may approach warning criteria, for example 50mm in 6 hours or 100mm in 24 hours. In addition, there is a risk that rainfall intensities could briefly reach 40mm in one hour.

The MetService says the source of the heavy rain is the very moist, sub-tropical air mass expected to spread across the North Island from the north Tasman Sea.

But before the showers and potentially heavy bursts of rain are unleashed on the middle and upper North Island, many parts of that area and much of the rest of New Zealand will be enjoying a sunny Saturday morning.

The rain and showers, like the high humidity in northern regions that have foreshadowed them, come from the moisture-laden air directed down over the country by ex-cyclone Winston.

"The humid northerly air flow behind ex-cyclone Winston means sub-tropical air that normally lies between Northland and Fiji is now starting to spill down over the nation," says WeatherWatch.

"It means the next couple of days -- and nights -- will be warm and muggy, particularly in northern coastal areas as this flow peaks."

As this warm, moist flow hits cooler southern air over the North Island later today, and tomorrow and Monday, the meeting could cause localised downpours, in a weather system whose progress will be slowed by a blocking high-pressure zone to the east of New Zealand.

Today's MetService forecast:

Auckland:

27C. Showers from afternoon, some heavy. Muggy northerlies

Hamilton: 28C. Rain from afternoon, possibly heavy. Muggy northerlies

Wellington: 21C. Cloudy. A few afternoon showers in the north. Gusty northerly